1. Field of the Invention
The present invention relates to an exposure control circuit for controlling the shutter time or the diaphragm digitally.
2. Description of the Prior Art
Recently various automatic exposure control devices with digital processing have been proposed, whereby the exposure factor such as the brightness information of the object to be photographed is digitally processed and the exposure condition such as the shutter time, the diaphragm value or the like is automatically determined so as to carry out the control. Hereby the processing is carried out digitally in these automatic exposure control device so that during the processing, the accuracy is increased and the control of the shutter time, the diaphragm value or the like can be carried out with very high accuracy. However, the objects such as the diaphragm, the shutter or the like to be controled by the above mentioned digital processing in a camera are mechanically composed members accordingly, despite the accurate processing result, errors often take place in the actual value of the shutter time, the diaphragm or the like due to the errors such as the delay of the response in the mechanical members themselves. Thus, despite the digital processing with high accuracy, the actual value of the shutter time, the diaphragm or the like becomes inaccurate. This is very inconvenient.
For example, in the camera which works with a focal plane shutter, it is so designed that at winding up the focal planes, the front and the rear focal plane pass by the aperture in such a manner that the both planes overlap above each other with a certain width so that, during the winding up operation, there takes place no space between the front and the rear plane. Thus, it is generally the case that the overlapping of the front and the rear plane remains with a proper width even after the completion of the winding up. From this state, the front plane is actuated by means of the shutter releasing and then the rear plane runs so as to complete the exposure with the desired shutter time. In consequence, the starting position of the front plane does not coincide with the starting position of the rear plane in most cases by the width of the overlapping so that only by controlling the time between the start of the front plane and that of the rear plane the time necessary for the planes to run by the width of the overlapping is disregarded, which causes an error in the actual shutter time.
Further, in the case where the diaphragm member is controlled in the diaphragm control device, which is so designed that the diaphragm member is closed from the maximum value toward the minimum value while producing pulses corresponding to the diaphragm value in such a manner that when the number of pulses coincide with the number of the pulses corresponding to a certain predetermined diaphragm value, the electromagnet is excited so as to stop the closing operation of the diaphragm member and determine the diaphragm value. Theoretically, the diaphragm member can be controlled at the predetermined diaphragm value if the closing operation of the diaphragm member is stopped immediately after the pulses whose number corresponds to the above mentioned diaphragm value are produced. Actually, after the pulses whose number corresponds to the diaphragm value are produced, the closing operation of the diaphragm member is stopped so that a certain determined delay of the response takes place, for example, by the time necessary for exciting the electromagnet and therefor the diaphragm can not be set at the predetermined diaphragm value.